String literals are normally enclosed in single quotation marks (') or double quotation marks (").
Single quotation marks are used to represent the exact text of the literal string; double quotation marks are used to interpolate the contents of the string.

#!/usr/bin/perl
$hi = 'Hi!';
print 'This is in single quotes: $hi'; 
print "\n";

#!/usr/bin/perl

$hi = 'Hi!';
print "This is in double quotes: $hi"; 
print "\n";

In addition to the quotation marks themselves, there are also operators that you can use to specify your own quote characters. In short, the q operator specifies a character that works like a single quotation mark, and the qq operator specifies a character that works like a double quotation mark. These operators are covered in more detail in the "String Operators" lesson of the next module, but they essentially work like this:

$x =q(This uses parenthesis as single quotes);
$x = qq|This uses vertical bars as double quotes|;

When characters that have matched pairs are used (for example, parenthesis, brackets, and so on), the matching character is used to close the quotes; otherwise the same character is used on both sides.
For example, if you have some text that you want to use, but the text itself includes quotation marks, it can be convenient to use the quote operators to delimit the text:

$phrase =
  q{"Four score and seven years ago," said Abe.};